Soap Lye Calculator
Lye and water for soap: NaOH or KOH across 7 oils with superfat.
What it does
Soap is lye meeting fat. Each oil needs a fixed lye share, its SAP value, to turn fully into soap plus glycerin. Too little lye leaves grease soup; too much leaves caustic bars. The calculator exists to land between them with a deliberate margin called superfat.
The tool above handles seven common oils in ounces or grams. Pick the oil, weight, superfat, and lye type. It returns lye weight, water at 38 percent of oils, and the recipe echo. Defaults of 32 oz olive at 5 percent open solved at 4.12 oz NaOH in 12.2 oz water.
SAP values descend from the saponification value: milligrams of KOH to saponify one gram of oil, per AOCS method Cd 3-25. Dividing KOH SAP by 1.403, the molecular weight ratio 56.11 over 40.00, gives NaOH SAP. Olive at 0.1355 needs far less lye than coconut at 0.1832, which is why coconut bars clean hard and olive bars stay mild.
Superfat is honesty about spread. Published SAP spans a range by crop and supplier, olive runs 184 to 196, and calculators plant their flag at midpoints. Five percent leftover oil swallows that uncertainty plus scale error, leaving conditioned bars instead of zappy ones.
Use the table to compare oils per kilogram, then run the recipe. Confirm exotic butters with the supplier, wear goggles and gloves, and always add lye to water, never water to lye.
How to use
- Pick imperial (oz) or metric (g).
- Pick the oil from the list.
- Enter total oil weight.
- Set superfat, 5% unless the recipe says otherwise.
- Pick NaOH for bars or KOH for liquid.
- Weigh lye and water separately, then mix lye into water.
Formula
- Oil
- Oil weight: oz imperial, g metric.
- SAP
- NaOH per gram of the picked oil.
- Superfat
- Leftover oil percent, default 5.
- Lye type
- NaOH bar soap or 90% KOH liquid soap.
SAP is grams of NaOH per gram of oil: olive 0.1355, coconut 76 deg 0.1832, palm 0.1419, lard 0.1412, shea 0.1276, cocoa 0.1383, castor 0.1283 (KOH values divided by 1.403). KOH SAP is 1.403 times NaOH from the molecular weights 56.11 and 40.00. Water defaults to 38% of oil weight.
Worked example
Example: 2 lb olive oil soap, 5% superfat
32 oz of olive oil at 5% superfat with NaOH. Find lye and water.
- SAP for olive is 0.1355 g NaOH per g oil.
- Full lye: 32 x 0.1355 = 4.336 oz.
- Superfat 5%: 4.336 x 0.95 = 4.12 oz NaOH.
- Water at 38% of oils: 32 x 0.38 = 12.2 oz.
- Dissolve 4.12 oz lye into 12.2 oz water, never the reverse.
- Metric mirror: 907 g olive gives 116.8 g NaOH in 344.7 g water.
Reference table
| Oil | SAP NaOH | Lye per kg | Note |
|---|---|---|---|
| Olive oil | 0.1355 | 128.7 g | Mild baseline (worked oil) |
| Coconut 76 deg | 0.1832 | 174 g | Hard cleansing bar |
| Palm oil | 0.1419 | 134.8 g | Firmness builder |
| Shea butter | 0.1276 | 121.2 g | Conditioning butter |
Limits
- Single oil only; blends need weighted SAP.
- Midpoint SAP values; supplier spread remains.
- 38% water is a default, not a rule.
- Safety gear and ventilation still required.
Common mistakes
- Adding water to lye instead of lye to water.
- Confusing NaOH with KOH.
- Weighing lye by volume.
- Running 0% superfat with midpoint SAP.
- Swapping coconut 76 with fractionated (0.325).
Try it yourself
Quick checks. Answers sit tucked under each question.
16 oz coconut, 0% superfat, KOH?
NaOH full: 16 x 0.1832 = 2.931 oz. KOH SAP is 1.403 times, purity 0.9: 2.931 x 1.403 / 0.9 = 4.57 oz KOH. Water 16 x 0.38 = 6.1 oz.
500 g shea butter, 5% superfat, NaOH?
Full: 500 x 0.1276 = 63.8 g. At 5%: 63.8 x 0.95 = 60.6 g NaOH. Water 500 x 0.38 = 190 g.
Why does metric match imperial?
907 g is 32 oz, and 0.1355 x 0.95 matches both paths: 116.8 g equals 4.12 oz.
Questions
NaOH or KOH?
Sodium hydroxide for solid bar soap, potassium hydroxide (90% flakes) for soft or liquid soap. The page handles both.
What is superfat?
Deliberate leftover oil, here 5%. It guarantees no free lye from SAP spread and conditions the bar.
How do KOH and NaOH SAP relate?
Roughly 0.713 of NaOH weight: KOH SAP is 1.403 times NaOH because KOH is the heavier molecule. The page converts internally.
How much water?
38% of oil weight here, a common beginner default. Advanced makers use lye concentration (often 33%) or water to oil ratio instead.
Lye safety in one line?
Always lye into water, goggles and gloves on, vinegar free zone, ventilated space. Lye burns are alkali and deepen with time.
Do calculators disagree?
Yes. SAP spans a range by crop and supplier (olive 184 to 196), and calculators pick midpoints. Superfat absorbs the spread; confirm exotic oils with the supplier.
Sources
- AOCS Official Method Cd 3-25: saponification value of fats and oils
- University of Washington Chemistry: saponification lecture demo